Soil type effects on soybean crop water use in weighing lysimeters

Summary In a previous experiment, evaporation from soybeans (Glycine max L.) in two weighing lysimeters with different soil types was found to differ by up to 30%. This occurred despite good canopy development and maintenance of well watered conditions. The present experiment sought to repeat the previous observation and to define its cause. Soybeans were sown in and around the two weighing lysimeters on 9 December 1987 and were well watered through the entire season. The lysimeters, L1 and L2 contained undisturbed blocks of Hanwood loam and Mundiwa clay loam soils, respectively, both Rhodoxeralfs. Crop growth, radiant energy interception, soil heat flux, canopy temperature and root growth were monitored through the season. Plant growth in L2 was slower than in L1 such that by 46 days from sowing (DFS), L1 plants had one leaf more on average than those in L2 and by 76 DFS plants in L2 were about 0.1 m shorter than those in either L1 or in the area immediately surrounding it. The ratio of L2 to L1 daily evaporation was 0.76 during the period 75 to 84 DFS; this being very similar to the effect observed previoulsy. The crop canopy in a 100 m² area centred around L2 was reduced in height by removing the top 0.15 m at 85 DFS. This treatment caused the L2L1 evaporation ratio to increase to 1.07. The effect of reducing the height of plants surrounding L2 was to increase net radiant energy intercepted in the canopy of the L2 plants and to change the turbulent transfer processes over the L2 canopy. Shading from the taller surrounding plants was estimated to have reduced evaporation by 4% while increased aerodynamic resistance above the L2 canopy as the result of the height discontinuity accounted for a further 20% reduction. This study highlights limitations in the application of one dimensional energy balance theory to non-ideal canopy configurations and to the care needed to ensure plant growth within lysimeters is the same as the surrounds.Visiting scientist     

Effects of soil type on soybean crop water use in weighing lysimeters

Summary Different soils are known to affect the amount and distribution of both available water and roots. Optimising irrigation water use, especially when shallow water-tables are present requires accurate knowledge of the root zone dynamics. This study was conducted to determine the effect of two soil types on root growth, soil water extraction patterns, and contributions of a water-table to crop evaporation (E). Two weighing lysimeters (L1 and L2) with undisturbed blocks of soil were used. The soil in L1 had higher hydraulic conductivity and lower bulk density than that in L2. Well watered conditions were maintained by irrigation for the first 110 days from sowing (DFS). Root length density (RLD) was calculated from observations made in clear acrylic tubes installed into the sides of the lysimeters. Volumetric soil water contents were measured with a neutron probe. A water-table (EC = 0.01 S m^-1) was established 1 m below the soil surface 18 DFS. RLD values were greater in L1 than L2 at any depth. In L1, maximum RLD values (3 × 10⁴ m m^-3) were measured immediately above the water-table at physiological maturity (133 DFS). In L2, maximum RLD values (1.5 × 10⁴ m m^-3) were measured at 0.42 m on 120 DFS and few roots were present above the water-table. From 71 to 74 DFS, 55 and 64% of E was extracted from above 0.2 m for L1 and L2, respectively. In L2, extraction was essentially limited to the upper 0.4 m, while L1 extraction was to 0.8 m depth. Around 100 DFS the water-table contributed 29% (L1) and 7% (L2) of the water evaporated. This proportion increased rapidly as the upper soil layers dried following the last substantial irrigation 106 DFS. Over the whole season the water-table contributed 24% in L1 and 6.5% in L2 of total E.     

覆膜滴灌对玉米田间水热传输及耗水的影响

基于充分滴灌条件下,定量分析了东北典型区玉米膜下滴灌与不覆膜滴灌下太阳净辐射、土壤热通量、土壤温度和作物耗水量的差异.结果表明:在晴天覆膜使得净辐射峰值时间滞后约1 h,在阴天覆膜处理土壤热通量能够为玉米生长提供更多的能量;全生育覆膜与不覆膜处理,冠层上方获得净辐射分别为133.25和135.42 W/m²,覆膜处理获得的可供能量低于不覆膜处理的,但在玉米生长中后期,覆膜处理提高了2%的净辐射;全生育期覆膜提高了0~80 cm土层平均地温1.13 ℃和土壤积温149.68 ℃;覆膜滴灌下玉米生长初期和后期的耗水量显著低于不覆膜滴灌处理下的,全生育期耗水量显著减少10.21%,生育期内平均作物系数K_c显著降低7.42%.     

A reexamination of the crop water stress index

Summary Hand-held infrared radiometers, developed during the past decade, have extended the measurement of plant canopy temperatures from individual leaves to entire plant canopies. Canopy temperatures are determined by the water status of the plants and by ambient meteorological conditions. The crop water stress index (CWSI) combines these factors and yields a measure of plant water stress. Two forms of the index have been proposed, an empirical approach as reported by Idso et al. (1981), and a theoretical approach reported by Jackson et al. (1981). Because it is simple and requires only three variables to be measured, the empirical approach has received much attention in the literature. It has, however received some criticism concerning its inability to account for temperature changes due to radiation and windspeed. The theoretical method is more complicated in that it requires these two additional variables to be measured, and the evaluation of an aerodynamic resistance, but it will account for differences in radiation and windspeed. This report reexamines the theoretical approach and proposes a method for estimating an aerodynamic resistance applicable to a plant canopy. A brief history of plant temperature measurements is given and the theoretical basis for the CWSI reviewed.     

Irrigation water distribution and long-term effects on crop and environment

The response of three water delivery schedules, representing various levels of flexibility, on crop production, water saving, soil salinization, drainage volumes and watertable behavior was examined. A physical-based transient soil water and solute transfer model, Soil–Water–Atmosphere–Plant (SWAP), was used as a tool. The evaluations were made for un-restricted and restricted water supply situations considering three different watertable conditions prevailing in the fourth drainage project (FDP) of the Punjab, Pakistan. From the simulation results it is apparent that on average the effect of irrigation schedule flexibility on crop yields is not very significant. However, compared to a fixed schedule provided un-restricted canal water supplies are available, the productivity of irrigation water supply (Y_act/I_rr), is up to 30% higher for the on-demand schedule. The on-demand schedule capable of complying with the temporal variations in climate is also more effective in water saving, reducing drainage volumes and controlling rising watertables if farmers follow guidelines and do not over-irrigate. In the present water deficient environment of the Indus basin, the benefits of the on-demand schedule and a fixed schedule are comparable. In the absence of sufficient canal water supplies, infrastructure and a well-designed and effective monitoring and communication system, moving towards the on-demand system will be un-productive. For the long-term sustainability of the irrigation system, improvements in the performance of the present water allocations and on-farm water management practices seems to be more necessary.     

含盐土壤不同作物根系分布对水盐分布的影响

以相同含盐水平农田种植的紫花苜蓿与玉米为研究对象,研究根系分布变化对水盐分布的影响,为盐渍化土壤紫花苜蓿合理化种植提供参考.结果表明:整个生育期0~100 cm苜蓿平均土壤含水率显著低于玉米,深层60~100 cm土壤平均含水率比玉米低1.98%,其有效利用深层水分.土壤平均储盐量随时间上下波动逐渐减小,二者盐分分布规律不同,苜蓿8月前后浅层土壤盐分均匀分布,玉米主要体现在8月后期.苜蓿脱盐率显著高于玉米,生长后期苜蓿深层根系占比增大,根系分布密集土层,脱盐效果明显.苜蓿细根根长密度与HCO₃^-呈正相关关系,HCO₃^-随根长密度增大而增大,土壤中HCO₃^-可减缓K⁺+Na⁺,Cl^-增加,使作物土壤达到降盐效果.     

Effect of shallow groundwater table on crop water requirements and crop yields

Due to the increasing demand for food and fiber by its ever-increasing population, the pressure on fresh water resources of Pakistan is increasing. Optimum utilization of surface and groundwater resources has become extremely important to fill the gap between water demand and supply. At Lahore, Pakistan 18 lysimeters, each 3.05 m × 3.05 m × 6.1 m deep were constructed to investigate the effect of shallow water tables on crop water requirements. The lysimeters were connected to bottles with Marriotte siphons to maintain the water tables at the desired levels and tensiometers were installed to measure soil water potential. The crops studied included wheat, sugarcane, maize, sorghum, berseem and sunflower. The results of these studies showed that the contribution of groundwater in meeting the crop water requirements varied with the water-table depth. With the water table at 0.5 m depth, wheat met its entire water requirement from the groundwater and sunflower absorbed more than 80% of its required water from groundwater. Maize and sorghum were found to be waterlogging sensitive crops whose yields were reduced with higher water table. However, maximum sugarcane yield was obtained with the water table at or below 2.0 m depth. Generally, the water-table depth of 1.5–2.0 m was found to be optimum for all the crops studied. In areas where the water table is shallow, the present system of irrigation supplies and water allowance needs adjustments to avoid over irrigation and in-efficient use of water.     

作物水氮耦合对农田土壤环境的影响

作物所处的土壤环境对提高作物产量和品质至关重要。为深入研究土壤生产力可持续发展提供理论依据,迫切需要了解现阶段作物水氮耦合效应对土壤环境的影响,重点总结了作物水氮耦合效应对土壤容重、孔隙度、含水量、温度、养分、有机质、酶活性、微生物和气体排放等土壤环境的影响,并提出未来水氮耦合的重点研究方向,以期为土壤的高效生产和可持续利用提供理论依据。      

Plant indicators of wheat and soybean crop water stress

Summary The onset of water stress within a crop is defined as the time at which the rate of water loss declines below that of a well watered crop in the same locality. The relation to the onset of water stress and soil water status of several readily measured plant parameters was investigated in crops of wheat and soybeans over three years. Evapotranspiration ET was monitored with weighing lysimeters. A noticeable decline in the rate of ET for both wheat and soybeans was detected once 20% to 30% of the total plant available water PAW remained in the 1 m deep lysimeter soil profile. Extension growth of wheat declined when PAW was 33% and 34% in two years of measurement. In soybeans, the decline in the rate of leaf extension coincided with the decline in the rate of ET. Midmorning measurement of exposed leaf water potential _L, covered leaf water potential _CL and covered plant leaf water potential _CP yielded similar results for both wheat and soybeans. Day-to-day variability was least in _CP and most in _L. Values of _CP, _L and _CL decreased rapidly with PAW < 30%. Daily values of leaf diffusive conductance were variable but there was a general decline in conductance with PAW < 30%. It is suggested that _CL may be the easiest and most reliable parameter to monitor as a means of detecting the onset of stress. The results indicated that PAW levels in the root zone of 50% for wheat and 30% for soybean probably do not affect extension growth or plant water status parameters and can thus be used as criteria for irrigation scheduling.Seconded from the Water Research Commission, Pretoria; present address: CSIRO, Division of Irrigation Research, Griffith, N SW 2680, Australia     

The effect of soil surface temperature on the crop water stress index

Summary A coupled soil-vegetation energy balance model which treats the canopy foliage as one layer and the soil surface as another layer was validated againt a set of field data and compared with a single-layer model of a vegetation canopy. The two-layer model was used to predict the effect of increases in soil surface temperature (T _s ) due to the drying of the soil surface, on the vegetation temperature (T _v ). In the absence of any change in stomatal resistance the impact of soil surface drying on the Crop Water Stress Index (CSWI) calculated from T _v was predicted. Field data came from a wheat crop growing on a frequently irrigated plot (W) and a plot left un watered (D) until the soil water depletion reached 100 mm. Vegetation and soil surface temperatures were measured by infrared thermometers from tillering to physiological maturity, with meteorological variables recorded simultaneously. Stomatal resistances were measured with a diffusion porometer intensively over five days when the leaf area index was between 5 and 8. The T _v predicted by the single-layer and the two-layer models accounted for 87% and 88% of the variance of measured values respectively, and both regression lines were close to the 11 relationship. Study of the effect of T _s on the CWSI with the two-layer model indicated that the CWSI was sensitive to changes in T _s . The overestimation of crop water stress calculated from the CWSI was predicted to be greater at low leaf area indices and high levels of stomatal resistance. The implications for this bias when using the CWSI for irrigation scheduling are discussed.List of Symbols C Sensible heat flux from the soil-vegetation system (W m^–2) - c _{l shade} Mean stomatal conductance of the shaded leaf area (m s^–1) - c _{l sun} Mean stomatal conductance of the sunlit leaf area (m s^–1) - c _max Maximum stomatal conductance (m s^–1) - c ₀ Minimum stomatal conductance (m s^–1) - C _p Specific heat at constant pressure (J kg^–1 °C^–1) - C _s Sensible heat flux from the soil (W m^–2) - C _v Sensible heat flux from the vegetation (W m^–2) - c _v Bulk stomatal conductance of the vegetation (m s^–1) - CWSI Crop Water Stress Index (dimensionless) - e _a Vapor pressure at the reference height (kPa) - e _b Vapor pressure at the virtual source/sink height of heat exchange (kPa) - e ₀ ^* Saturated vapor pressure at T ₀ (kPa) - e _s Vapor pressure at the soil surface (kPa) - e _v ^* Saturated vapor pressure at T _v (kPa) - G Soil heat flux (Wm^–2) - GLAI Green leaf area index (dimensionless) - GLAI_shade Green shaded leaf area index (dimensionless) - GLAI_sun Green sunlit leaf area index (dimensionless) - k Extinction coefficient for photosynthetically active radiation (dimensionless) - k ₁ Damping exponent for Eq. A 5 (m² W^–1) - LAI Leaf area index (dimensionless) - LE Latent heat flux from the soil-vegetation system (W m^–2) - LE _s Latent heat flux from the soil (W m^–2) - LE _v Latent heat flux from the vegetation (W m^–2) - p _a Density of air (kg m^–3) - PAR_a Photosynthetically active radiation above the canopy (W m^–2) - PAR_u Photosynthetically active radiation under the canopy (W m^–2) - r _a Aerodynamic resistance (s m^–1) - r _b Heat exchange resistance between the vegetation and the adjacent air boundary layer (s m^–1) - r _c Bulk stomatal resistance of the vegetation (s m^–1) - R _n Net radiation above the canopy (W m^–2) - R _s Net radiation flux at the soil surface (W m^–2) - r _st Mean stomatal resistance of leaves in the canopy (s m^–1) - R _v Net radiation absorbed by the vegetation (W m^–2) - r _w Heat exchange resistance between the soil surface and the boundary layer (s m^–1) - S Photosynthetically active radiation on the shaded leaves (W m^–2) - S _d Diffuse photosynthetically active radiation (W m ^–2) - S ₀ Photosynthetically active radiation on a surface perpendicular to the beams (W m^–2) - T _a Air temperature at the reference height (°C) - T _b Temperature at the virtual source/sink height of heat exchange (°C) - T ₀ Aerodynamic temperature (°C) - T _s Soil surface temperature (°C) - T _v Vegetation temperature (°C) - w ₀ Single scattering albedo (dimensionless) - Psychrometric constant (kPa °C) - ₀ Cosine of solar zenith angle (dimensionless)     

Effect of seasonal water stress imposed on drip irrigated second crop watermelon grown in semi-arid climatic conditions

A study was conducted to determine the water stress effect on yield and some physiological parameters including crop water stress index for drip irrigated second crop watermelon. Irrigations were scheduled based on replenishment of 100, 75, 50, 25, and 0% soil water depletion from 90 cm soil depth with 3-day irrigation interval. Seasonal crop evapotranspiration (ET) for I₁₀₀, I₇₅, I₅₀, I₂₅, and I₀ were 660, 525, 396, 210, and 70 mm in 2003 and 677, 529, 405, 221, and 75 mm in 2004. Fruit yield was significantly lowered by irrigation water stress. Average water-yield response factor for both of the years was 1.14. The highest yield was obtained from full irrigated treatment as 34.5 and 38.2 t ha⁻¹ in 2003 and 2004, respectively. Lower ET rates and irrigation amounts in water stress treatments resulted in reductions in all measured parameters, except water-soluble dry matter concentrations (SDM). Canopy dry weights, leaf relative water content, and total leaf chlorophyll content were significantly lowered by water stress. Yield and seasonal ET were linearly correlated with mean CWSI values. An average threshold CWSI value of 0.17 before irrigation produced the maximum yield and it could be used to initiate the irrigation for watermelon.     

Effect of field levelling quality on irrigation efficiency and crop yield

Proper land levelling in areas irrigated by canals is an essential pre-requisite for judicious use of scarce irrigation water. The concept of land levelling index, which is different from land uniformity coefficient used hitherto, has been adopted to evaluate quantitatively the precision or quality of land levelling work. Using this concept the quality of land levelling work being done in newly canal commanded areas has been assessed and it is demonstrated that the prevalent deficiency in levelling quality ranges from 21.4 to 42.2%. Field investigations have been carried out to assess quantitatively the effect of land levelling quality on irrigation and water-use efficiencies and crop yield. The study comprised five levelling indices viz., 1.2, 2.0, 2.5, 3.0 and 3.7 cm and two irrigation treatments, i.e., fixed depth and complete irrigation. The results show that irrigation application, distribution, and water-use efficiency are appreciably reduced with increase in levelling index or deterioration in levelling quality. The wheat grain yield is also similarly reduced and it is significantly less for levelling index values higher than 2.5 cm. The fixed irrigation depth treatment gives better results from the point of water economy than does the complete irrigation treatment. From this study, it can be inferred that to obtain reasonably high yield and judicious use of water at a reasonable cost the field levelling index should not have a value greater than 3.0 cm.     

Daylight crop water stress index for continuous monitoring of water status in apple trees

We studied the suitability of empirical crop water stress index (CWSI) averaged over daylight hours (CWSI_d) for continuous monitoring of water status in apple trees. The relationships between a midday CWSI (CWSI_m) and the CWSI_d and stem water potential (ψ _stem), and soil water deficit (SWD) were investigated. The treatments were: (1) non-stressed where the soil water was close to field capacity and (2) mildly stressed where SWD fluctuated between 0 and a maximum allowable depletion (MAD of 50 %). The linear relationship between canopy and air temperature difference (ΔT) and air vapor pressure deficit (VPD) averaged over daylight hours resulted in a non-water-stressed baseline (NWSBL) with higher correlation (?T = ?0.97 VPD – 0.46, R ² = 0.78, p < 0.001) compared with the conventional midday approach (?T = ?0.59 VPD – 0.67, R ² = 0.51, p < 0.001). Wind speed and solar radiation showed no significant effect on the daylight NWSBL. There was no statistically meaningful relationship between midday ψ _stem and CWSI_m. The CWSI_d agreed well with SWD (R ² = 0.70, p < 0.001), while the correlation between SWD and CWSI_m was substantially weaker (R ² = 0.38, p = 0.033). The CWSI_d exhibited high sensitivity to mild variations in the soil water content, suggesting it as a promising indicator of water availability in the root zone. The CWSI_d is stable under transitional weather conditions as it reflects the daily activity of an apple crop.     

Development of crop water stress index of wheat crop for scheduling irrigation using infrared thermometry

This study was conducted to develop the relationship between canopy-air temperature difference and vapour pressure deficit for no stress condition of wheat crop (baseline equations), which was used to quantify crop water stress index (CWSI) to schedule irrigation in winter wheat crop (Triticum aestivum L.). The randomized block design (RBD) was used to design the experimental layout with five levels of irrigation treatments based on the percentage depletion of available soil water (ASW) in the root zone. The maximum allowable depletion (MAD) of the available soil water (ASW) of 10, 40 and 60 per cent, fully wetted (no stress) and no irrigation (fully stressed) were maintained in the crop experiments. The lower (non-stressed) and upper (fully stressed) baselines were determined empirically from the canopy and ambient air temperature data obtained using infrared thermometry and vapour pressure deficit (VPD) under fully watered and maximum water stress crop, respectively. The canopy-air temperature difference and VPD resulted linear relationships and the slope (m) and intercept (c) for lower baseline of pre-heading and post-heading stages of wheat crop were found m = −1.7466, c = −1.2646 and m = −1.1141, c = −2.0827, respectively. The CWSI was determined by using the developed empirical equations for three irrigation schedules of different MAD of ASW. The established CWSI values can be used for monitoring plant water status and planning irrigation scheduling for wheat crop.     

Evaluation of crop water stress index for LEPA irrigated corn

This study was designed to evaluate the crop water stress index (CWSI) for low-energy precision application (LEPA) irrigated corn (Zea mays L.) grown on slowly-permeable Pullman clay loam soil (fine, mixed, Torrertic Paleustoll) during the 1992 growing season at Bushland, Tex. The effects of six different irrigation levels (100%, 80%, 60%, 40%, 20%, and 0% replenishment of soil water depleted from the 1.5-m soil profile depth) on corn yields and the resulting CWSI were investigated. Irrigations were applied in 25 mm increments to maintain the soil water in the 100% treatment within 60–80% of the “plant extractable soil water” using LEPA technology, which wets alternate furrows only. The 1992 growing season was slightly wetter than normal. Thus, irrigation water use was less than normal, but the corn dry matter and grain yield were still significantly increased by irrigation. The yield, water use, and water use efficiency of fully irrigated corn were 1.246 kg/m², 786 mm, and 1.34 kg/m³, respectively. CWSI was calculated from measurements of infrared canopy temperatures, ambient air temperatures, and vapor pressure deficit values for the six irrigation levels. A “non-water-stressed baseline” equation for corn was developed using the diurnal infrared canopy temperature measurements as T _c–T _a = 1.06–2.56 VPD, where T _c was the canopy temperature (°C), Ta was the air temperature (°C) and VPD was the vapor pressure deficit (kPa). Trends in CWSI values were consistent with the soil water contents induced by the deficit irrigations. Both the dry matter and grain yields decreased with increased soil water deficit. Minimal yield reductions were observed at a threshold CWSI value of 0.33 or less for corn. The CWSI was useful for evaluating crop water stress in corn and should be a valuable tool to assist irrigation decision making together with soil water measurements and/or evapotranspiration models. Received: 19 May 1998     

Energy balance and crop water stress in winter maize under phenology-based irrigation scheduling

In eastern India, cultivation of winter maize is getting popular after rainy season rice and farmers practice irrigation scheduling of this crop based on critical phenological stages. In this study, crop water stress index of winter maize at different critical stages wase determined to investigate if phenology-based irrigation scheduling could be optimized further. The components of the energy budget of the crop stand were computed. The stressed and non-stressed base lines were also developed (between canopy temperature and vapor pressure deficit) and with the help of base line equation, [(T _c − T _a) = −1.102 VPD − 3.772], crop water stress index (CWSI) was determined from the canopy-air temperature data collected frequently throughout the growing season. The values of CWSI (varied between 0.42 and 0.67) were noted just before the irrigations were applied at critical phenological stages. The soil moisture depletion was also measured throughout the crop growing period and plotted with CWSI at different stages. Study revealed that at one stage (silking), CWSI was much lower (0.42–0.48) than that of recommended CWSI (0.60) for irrigation scheduling. Therefore, more research is required to further optimize the phenology-based irrigation scheduling of winter maize in the region. This method is being used now by local producers. The intercepted photosynthetically active radiation and normalized difference vegetation index over the canopy of the crop were also measured and were found to correlate better with leaf area index.     

Application of a new method to evaluate crop water stress index

Optimum water management and irrigation require timely detection of crop water condition. Usually crop water condition can be indicated by crop water stress index (CWSI), which can be estimated based on the measurements of either soil water or plant status. Estimation of CWSI by canopy temperature is one of them and has the potential to be widely applied because of its quick response and remotely measurable features. To calculate CWSI, the conventional canopy-temperature-based model (Jackson’s model) requires the measurement or estimation of the canopy temperature, the maximum canopy temperature (T _cu), and the minimum canopy temperature (T _cl). Because extensive measurements are necessary to estimate T _cu and T _cl, its application is limited. In this study, by introducing the temperature of an imitation leaf (a leaf without transpiration, T _p) and based on the principles of energy balance, we studied the possibility to replace T _cu by T _p and reduce the included parameters for CWSI calculation. Field experiments were carried out in a winter wheat (Triticum aestivum L.) field in Luancheng area, Hebei Province, the main production area of winter wheat in China. Six irrigation treatments were established and soil water content, leaf water potential, soil evaporation rate, plant transpiration rate, biomass, yield, and regular meteorological variables of each treatment were measured. Results indicate that the values of T _cu agree with the values of T _p with a regression coefficient r=0.988. While the values of CWSI estimated by the use of T _p are in agreement with CWSI by Jackson’s method, with a regression coefficient r=0.999. Furthermore, CWSI estimated by the use of T _p has good relations with soil water content and leaf water potential, showing that the estimated CWSI by T _p is a good indicator of soil water and plant status. Therefore, it is concluded that T _cu can be replaced by T _p and the included parameters for CWSI calculation can be significantly reduced by this replacement.     

Canopy temperature variability as an indicator of crop water stress severity

Irrigation scheduling requires an operational means to quantify plant water stress. Remote sensing may offer quick measurements with regional coverage that cannot be achieved by current ground-based sampling techniques. This study explored the relation between variability in fine-resolution measurements of canopy temperature and crop water stress in cotton fields in Central Arizona, USA. By using both measurements and simulation models, this analysis compared the standard deviation of the canopy temperature to the more complex and data intensive crop water stress index (CWSI). For low water stress, field was used to quantify water deficit with some confidence. For moderately stressed crops, the was very sensitive to variations in plant water stress and had a linear relation with field-scale CWSI. For highly stressed crops, the estimation of water stress from is not recommended. For all applications of one must account for variations in irrigation uniformity, field root zone water holding capacity, meteorological conditions and spatial resolution of T _c data. These sensitivities limit the operational application of for irrigation scheduling. On the other hand, was most sensitive to water stress in the range in which most irrigation decisions are made, thus, with some consideration of daily meteorological conditions, could provide a relative measure of temporal variations in root zone water availability. For large irrigation districts, this may be an economical option for minimizing water use and maximizing crop yield.

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Using radiation thermography and thermometry to evaluate crop water stress in soybean and cotton

The use of digital infrared thermography and thermometry to investigate early crop water stress offers a producer improved management tools to avoid yield declines or to deal with variability in crop water status. This study used canopy temperature data to investigate whether an empirical crop water stress index could be used to monitor spatial and temporal crop water stress. Different irrigation treatment amounts (100%, 67%, 33%, and 0% of full replenishment of soil water to field capacity to a depth of 1.5 m) were applied by a center pivot system to soybean (Glycine max L.) in 2004 and 2005, and to cotton (Gossypium hirsutum L.) in 2007 and 2008. Canopy temperature data from infrared thermography were used to benchmark the relationship between an empirical crop water stress index (CWSI_e) and leaf water potential (Ψ_L) across a block of eight treatment plots (of two replications). There was a significant negative linear correlation between midday Ψ_L measurements and the CWSI_e after soil water differences due to irrigation treatments were well established and during the absence of heavy rainfall. Average seasonal CWSI_e values calculated for each plot from temperature measurements made by infrared thermometer thermocouples mounted on a center pivot lateral were inversely related to crop water use with r² values >0.89 and 0.55 for soybean and cotton, respectively. There was also a significant inverse relationship between the CWSI_e and soybean yields in 2004 (r² = 0.88) and 2005 (r² = 0.83), and cotton in 2007 (r² = 0.78). The correlations were not significant in 2008 for cotton. Contour plots of the CWSI_e may be used as maps to indicate the spatial variability of within-field crop water stress. These maps may be useful for irrigation scheduling or identifying areas within a field where water stress may impact crop water use and yield.     

微咸水灌溉下土壤水盐动态及对作物产量的影响

华北平原农业灌溉用水非常紧缺,水资源日益缺乏与粮食需求日益增多之间的矛盾尖锐。充分利用微咸水资源是缓解这一矛盾的重要途径之一。该文以中国农业大学曲周试验站1997－2005年冬小麦和夏玉米微咸水灌溉田间长期定位试验为基础,研究了充分淡水、充分淡咸水、关键期淡水、关键期淡咸水和不灌溉等5个处理下土壤饱和电导率和含盐量的动态变化,探讨了微咸水灌溉对冬小麦和夏玉米产量的影响。结果表明：土壤水盐动态呈受灌溉和降雨影响的短期波动和受季节更替影响的长期波动;在正常降雨年份,使用微咸水进行灌溉是可行的,不会导致土壤的次生盐渍化;微咸水灌溉虽然导致冬小麦和夏玉米产量降低10%～15%,但节约淡水资源60%～75%。如果降雨量达到多年平均水平以及微咸水灌溉制度制订合理,微咸水用于冬小麦/玉米田间灌溉前景广阔。